As a fixing device used for an electrophotographic image forming apparatus such as a copying machine, a facsimile, or a printer, a fixing device of a heat roller fixing type is adopted widely. The fixing device of the heat roller fixing type conveys a sheet to a nip portion formed by a fixing roller controlled at a predetermined temperature and a pressing roller in pressure contact with the fixing roller, heats and pressurizes a non-fixed toner image on the sheet surface, thereby fixes it.
However, widening the fixing nip portion is required to heat toner efficiently on the sheet surface by the demand for colorization and speedup in late years. However, viewing the constitution of the fixing device of a heat roller fixing type, a means for enlarging the diameters of the two rollers or increasing the pressurizing force between the rollers, thereby increasing the collapse (distortion) amount of the rollers would be considered to widen the nip portion. However, there may arise a problem in respect to the enlargement of the size of the fixing device and the reduction in durability of the fixing device and the degree of freedom of design conditions is low.
As a method for solving this problem, a fixing device of a fixing belt type having an rotating endless fixing belt driven by a fixing roller and a pressing pad fixed on the inner peripheral surface side of the fixing belt and pressing the fixing belt toward the fixing roller by the pressing pad has been adopted in recent years. In the fixing device of a fixing belt type, from the viewpoint of its constitution, the nip width can be set comparatively freely.
In the fixing device of a fixing belt type, the shape of the nip portion is in a circular arc shape along the fixing roller, so that defective separation of a sheet from the fixing roller is caused easily. Defective separation of a sheet is that the sheet passing the nip portion may be conveyed without separated normally from the fixing roller kept wound round it. It is easy to happen the defective separation of a sheet especially when a coated paper with high surface smoothness and a thin paper with low rigidity are used.
As a method for avoiding such defective separation of a sheet, Japanese Patent Application Publication NO. 5-150679 discloses a fixing device wherein a pressure roller is installed inside the fixing belt at the exit of the nip portion, and the fixing roller is distorted, thus the sheet peeling (separation) performance is improved.
However, when the pressure roller is installed and the fixing roller is distorted, a sheet conveying speed V2 at that portion (hereinafter, referred to as a second nip portion) is higher than a conveying speed V1 of the other part (hereinafter, referred to as a first nip portion) which is not distorted.
When the pressurizing force (total load) to the fixing roller by the fixing belt at the second nip position is excessively large, a total load P2 to the sheet at the concerned position becomes higher than a total load P1 to the sheet at the first nip portion. In this case, the conveying force at the second nip portion is higher than the conveying force at the first nip portion, so that the conveying force at the second nip portion is dominant and the sheet is conveyed at the conveying speed V2 at that position.
Therefore, the conveying speed for all the sheets is changed from the conveying speed V1 at the first nip portion to the conveying speed V2 at the second nip portion and the sheet conveying speed is not stabilized. In correspondence with such a change in the conveying speed, an image slipping occurs.
For such an image slipping, in the fixing device recorded in Japanese Patent Application Publication NO. 8-166734, at the first nip portion on the upstream side, a pressure auxiliary roll of low hardness is installed on the inner peripheral surface of the fixing belt and the total load P1 to the sheet at the first nip portion becomes higher than the total load P2 to the sheet at the second nip portion on the downstream side.
However, in the fixing device recorded in Japanese Patent Application Publication No. 8-166734, to improve the separation performance and simultaneously prevent an image slipping, it is necessary to increase the total load P2 to the fixing roller at the second nip portion and also increase the total load P1 at the first nip portion and in that case, a problem of an increase in the drive torque of the fixing device and a reduction in the durability of the fixing roller arises.
With the foregoing problem in view, It is an object of the present invention to provide a fixing device and an image forming apparatus for preventing an occurrence of an image slipping without increasing the total load P1 at the first nip portion, even when the total load P2 at the second nip portion on the exit side of the fixing nip is increased to enhance the sheet separation performance.